This invention relates to electrochemical cells and batteries, and more particularly, to improved electrode active material of such batteries, and novel methods of synthesis.
Lithium batteries are prepared from one or more lithium electrochemical cells containing electrochemically active (electroactive) materials. Such cells typically include an anode (negative electrode), a cathode (positive electrode), and an electrolyte interposed between spaced apart positive and negative electrodes. Batteries with anodes of metallic lithium and containing metal chalcogenide cathode active material are known. The electrolyte typically comprises a salt of lithium dissolved in one or more solvents, typically nonaqueous (aprotic) organic solvents. Other electrolytes are solid electrolytes typically called polymeric matrixes that contain an ionic conductive medium, typically a metallic powder or salt, in combination with a polymer that itself may be ionically conductive which is electrically insulating. By convention, during discharge of the cell, the negative electrode of the cell is defined as the anode. Cells having a metallic lithium anode and metal chalcogenide cathode are charged in an initial condition. During discharge, lithium ions from the metallic anode pass through the liquid electrolyte to the electrochemical active (electroactive) material of the cathode whereupon they release electrical energy to an external circuit.
It has recently been suggested to replace the lithium metal anode with an intercalation anode, such as a lithium metal chalcogenide or lithium metal oxide. Carbon anodes, such as coke and graphite, are also intercalation materials. Such negative electrodes are used with lithium-containing intercalation cathodes, in order to form an electroactive couple in a cell. Such cells, in an initial condition, are not charged. In order to be used to deliver electrochemical energy, such cells must be charged in order to transfer lithium to the anode from the lithium-containing cathode. During discharge the lithium is transferred from the anode back to the cathode. During a subsequent recharge, the lithium is transferred back to the anode where it reintercalates. Upon subsequent charge and discharge, the lithium ions (Li+) are transported between the electrodes. Such rechargeable batteries, having no free metallic species are called rechargeable ion batteries or rocking chair batteries. See U.S. Pat. Nos. 5,418,090; 4,464,447; 4,194,062; and 5,130,211.
Preferred positive electrode active materials include LiCoO2, LiMn2O4, and LiNiO2. The cobalt compounds are relatively expensive and the nickel compounds are difficult to synthesize. A relatively economical positive electrode is LiMn2O4, for which methods of synthesis are known, and involve reacting generally stoichiometric quantities of a lithium-containing compound and a manganese containing compound. The lithium cobalt oxide (LiCoO2), the lithium manganese oxide (LiMn2O4), and the lithium nickel oxide (LiNiO2) all have a common disadvantage in that the charge capacity of a cell comprising such cathodes suffers a significant loss in capacity. That is, the initial capacity available (amp hours/gram) from LiMn2O4, LiNiO2, and LiCoO2 is less than the theoretical capacity because less than 1 atomic unit of lithium engages in the electrochemical reaction. Such an initial capacity value is significantly diminished during the first cycle operation and such capacity further diminishes on every successive cycle of operation. The specific capacity for LiMn2O4 is at best 148 milliamp hours per gram. As described by those skilled in the field, the best that one might hope for is a reversible capacity of the order of 110 to 120 milliamp hours per gram. Obviously, there is a tremendous difference between the theoretical capacity (assuming all lithium is extracted from LiMn2O4) and the actual capacity when only 0.8 atomic units of lithium are extracted as observed during operation of a cell. For LiNiO2 and LiCoO2 only about 0.5 atomic units of lithium is reversibly cycled during cell operation. Many attempts have been made to reduce capacity fading, for example, as described in U.S. Pat. No. 4,828,834 by Nagaura et al. However, the presently known and commonly used, alkali transition metal oxide compounds suffer from relatively low capacity. Therefore, there remains the difficulty of obtaining a lithium-containing chalcogenide electrode material having acceptable capacity without disadvantage of significant capacity loss when used in a cell.
Capacity fading is well known and is calculated according to the equation given below. The equation is used to calculate the first cycle capacity loss. This same equation is also used to calculate subsequent progressive capacity loss during subsequent cycling relative back to the first cycle capacity charge reference.       (                  (                  FC          ⁢                      xe2x80x83                    ⁢          charge          ⁢                      xe2x80x83                    ⁢          capacity                )            -                        (                      FC            ⁢                          xe2x80x83                        ⁢            discharge            ⁢                          xe2x80x83                        ⁢            capacity                    )                xc3x97        100        ⁢        %                  FC    ⁢          xe2x80x83        ⁢    charge    ⁢          xe2x80x83        ⁢    capacity  
In U.S. Pat. No. 4,828,834 Nagaura et al. attempted to reduce capacity fading by sintering precursor lithium salt and MnO2 materials and thereby forming an LiMn2O4 intercalation compound. However, Nagaura""s LiMn2O4 compounds were not fully crystallized spinel electrodes and suffered from a very low capacity. Despite the above approaches, there remains the difficulty of obtaining lithium manganese oxide based electrode materials having the attractive capacity of the basic spinel LixMn2O4 intercalation compound, but without its disadvantage of significant capacity loss on progressive cycling.
The present invention provides a composition suitable for use as an electrochemically active material for an electrochemical cell. The composition comprises particles of spinel lithium manganese oxide having on the surface of the particles ionic metal species bound to the spinel at oppositely charged respective ionic sites of the spinel particle surface. The ionic metal species preferably includes a transition metal. Alternatively, the ionic metal species includes a non-transition metal capable of a +3 valence state. The ionic species may contain mixtures of the foregoing metals. Cationic metal species bound to the spinel particle surface include, but are not limited to, metal cation, metal oxide cation, and metal phosphate cation.
In a preferred method, the composition comprising the spinel lithium manganese oxide having ionic species bound thereto is prepared by decomposing or melting a precursor metal compound on the surface of the spinel particles, thereby giving rise to the cationic metal species.
The spinel lithium manganese oxide treated for improved results by the method of the invention is known to have the nominal formula Li1Mn2O4. Such spinel lithium manganese oxide compounds may vary in the relative proportion of lithium, manganese, and oxygen while maintaining identity as a spinel lithium manganese oxide insertion compound. The invention is not limited to any particular formulation for a spinel lithium manganese oxide. However, advantageous results are obtained when the spinel lithium manganese oxide is represented by the nominal formula Li1+xMn2xe2x88x92xO4 where x is a range of about xe2x88x920.2 to about +0.5; and more preferably where x is greater than zero and up to about 0.5.
The treated spinel lithium manganese oxide is prepared as an electrode by mixing it with a binder and optionally with an electrically conductive material and forming it into an electrical structure.
The composite spinel manganese oxide particles having the metal species bound thereto is prepared by first forming a mixture comprising the lithium manganese oxide particles and the metal compound. The mixture may be formed by mixing lithium manganese oxide powder and metal compound powder. Alternately, the metal compound (metal salt) is dissolved in a suitable solvent, then the lithium manganese oxide particles are thoroughly wetted by the solution before reaction. Reaction is conducted by heating the mixture containing the spinel particles and the metal compound for a time and at a temperature sufficient to form a decomposition product of the metal compound on the surface of the particles. In an alternative embodiment, the metal compound is reacted at the surface of the spinel while undergoing limited, very little, or no decomposition. For example, when the metal compound is a phosphate salt, the phosphate salts retain their phosphate groups, and the heating serves to chemically disperse and adhere the phosphate to the spinel.
It is preferred that the heating to cause reaction between the metal compound and the surface of the lithium manganese oxide be conducted at a temperature in a range from about 20xc2x0 C. to about 800xc2x0 C., desirably 200xc2x0 C. to about 750xc2x0 C., more desirably 2000xc2x0 C. to about 700xc2x0 C., in an air atmosphere for at least about xc2xd hour and up to about 6 hours. Since any amount of the metal compound will improve the characteristics of the LMO, there is no practical lower limit to the amount to be added as long as the amount is greater than zero. It is preferred that the amount of metal compound included with the lithium manganese oxide in the mixture be up to about 10 wt. % of the mixture with the lithium manganese oxide constituting the balance.
Objects, features, and advantages of the invention include an improved electrochemical cell or battery based on lithium which has improved charging and discharging characteristics, a large discharge capacity, and which maintains its integrity during cycling. Another object is to provide a cathode active material which combines the advantages of large discharge capacity and with relatively lesser capacity fading. It is also an object of the present invention to provide positive electrodes having active materials which operate with good performance over a relatively broad temperature range. Another object is to provide a method for forming cathode active material which lends itself to commercial scale production providing for ease of preparing large quantities.
These and other objects, features, and advantages will become apparent from the following description of the preferred embodiments, claims, and accompanying drawings.